This invention relates generally to ultrasound imaging systems and, more particularly, to an ultrasound transducer design that improves the resolution of an ultrasound imaging system.
Ultrasound transducers used for medical imaging and non-destructive testing are characterized by two main properties, sensitivity and bandwidth, which are directly correlated to the penetration and resolution of the imaging system. As transducer designs have become more sophisticated, fractional bandwidth has increased from 30-40% with a single matching layer to 60-80% with two matching layers. In medical diagnostic ultrasound imaging, recent advances in harmonic imaging, with and without contrast agents, have highlighted the benefits of even broader bandwidth probes. As implied by the name, harmonic imaging requires sensitivity at over 100% fractional bandwidth. The fractional bandwidth (FBW) is defined as the bandwidth divided by the center frequency:   FBW  =                    U        lim            -              L        lim                    f      ctr      
where Ulim is the upper limit of the bandwidth, Llim is the lower limit of the bandwidth, and fctr is the center frequency.
A very-wide-bandwidth probe can simplify scanning by reducing the number of probes needed to perform a diagnosis. In conventional situations, a high-frequency probe is needed to look for fine detail close to the skin and a lower-frequency probe is used for color flow imaging, Doppler imaging, and imaging at greater depths in the body. It takes time and operator motions to switch between probes. If this can be minimized, the time and effort required to complete a patient scan can be reduced.
Efficient ultrasound transducers have a very different acoustic impedance than most objects under test. This makes it difficult to couple ultrasound waves between these two materials. It is well known that acoustic matching layers improve the sensitivity and bandwidth of ultrasound transducers by more efficiently transmitting acoustic energy from materials of one specific acoustic impedance, such as piezoelectric ceramics, to materials of a different specific acoustic impedance, such as water baths or the human body.
The theory of acoustic matching layers is well understood and is very similar to electronic filter design methods, as disclosed by T. Rhyne in xe2x80x9cComputer Optiaebzation of Transducer Transfer Functions Using Constraints on Bandwidth, Ripple, and Loss,xe2x80x9d IEEE Trans. Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 43, No. 6, pp. 1136-1149 (November 1996). By adjusting the acoustic impedance and thickness of the matching layers, a variety of standard bandpass characteristics can be achieved, as disclosed in U.S. Pat. No. 5,706,564 to Rhyne. As shown in the Rhyne patent, a convenient continuum exists between the Thompson and Butterworth band shapes. The practical difficulty for ultrasound transducers is that the optimal acoustic impedances for many matching layers are not attainable with simple materials. For example, in two-layer designs the inner matching layer needs to have an impedance of about 7-10 Mrayls. This is inconveniently higher than plastics (2-4 Mrayls) and less than glasses and metals (10-100 Mrayls).
When trying to increase ultrasound transducer bandwidth, several approaches are possible. However, many involve complicated material developments such as the growth of large single-crystal piezoelectrics, or complex composite structures. See, for example, Park et al., xe2x80x9cCharacteristics of Relaxor-Based Piezoelectric Single Crystals for Ultrasonic Transducers,xe2x80x9d 1996 IEEE Ultrasonics Symposium, pp. 935-942 (1996), and W. A. Smith, xe2x80x9cThe Role of Piezocomposites in Ultrasound Transducers,xe2x80x9d 1989 IEEE Ultrasonics Symposium, pp. 755-766 (1989). Thus there is need for a simple matching layer material having optimal acoustic impedance and which can be easily processed.
An ultrasound transducer achieves significantly better resolution by fabricating the acoustic matching layer closest to the piezoelectric layer from silicon. Silicon is a simple, ubiquitous and readily available material which has been extensively studied, is inexpensive, and is relatively easy to process. Since silicon is the building block for essentially all semiconductor electronics, wafers up to many inches in diameter are readily available.
In the simplest embodiment, a silicon wafer as obtained for semiconductor processing is ground to an appropriate thickness, and included in the acoustic stack with other matching layer materials during transducer construction. The exact thickness is determined by the details of the design, but is nominally a quarter wavelength in the material.
In accordance with one preferred embodiment, the transducer matching layer structure is designed (using computer optimization) to produce a Chebyshev bandshape. For this bandshape, the ideal acoustic impedance for the third layer is about 18.6 Mrayls, so that the standard silicon wafer having an acoustic impedance of 19.6 Mrayls is nearly, but not quite, optimum. However, different shapes lead to modified values of the acoustic impedance. By changing the shape of the silicon to a narrow beam, for example, which is the appropriate shape for a transducer array element, a better match to the desired impedance is obtained.
In a further embodiment, the silicon wafer orientation can be selected to more accurately match the silicon acoustic impedance to that required for a specific bandshape design. Since silicon is a cubic material, three standard orientations or xe2x80x9ccutsxe2x80x9d designated by Miller indices (100), (110) and (111) are available. These indices are related to the orientation of the wafer plane relative to the crystal axes. In a more complex preferred embodiment, if alternative cuts are taken, even more flexibility in acoustic impedance is possible.